Wireless communication service providers offer many location-based services such as emergency services, mobile yellow pages, and navigation assistance to subscribers. In fact, service providers will be required to accurately locate subscribers requesting emergency assistance via 911 to comply with the regulations of the United States Federal Communications Commission (FCC) known as Enhanced 911 (E-911). To determine the location of a mobile station (e.g., a cellular telephone, a pager, or a handheld computer), a Global Positioning System (GPS) receiver can be used. In a GPS application, the location of the mobile station may be determined by using range information derived from the code phase corresponding to the peak of an auto correlation function (ACF). The ACF may be generated by correlating a received signal with a local replica of a pseudorandom code modulated on a carrier signal transmitted by from GPS satellites.
Typically, a conventional GPS receiver determines the code phase offset based on historical information. That is, the GPS receiver tracks pseudorandom codes from GPS satellites (i.e., satellite signals). Then, the GPS receiver compares a correlation magnitude derived using an earlier version of its replica code with a correlation magnitude derived using a later version of its replica code. The difference between the two correlation magnitudes is used to close a tracking loop (generally known as a delay lock loop (DLL) to keep the code phase estimate as close as possible to the peak of the ACF. Based on the code phase estimate, and a means to resolve the integer ambiguity associated with the code (i.e., the number of integer pseudorandom code lengths between the receiver and each satellite), position of the mobile station (i.e., latitude, longitude, and/or altitude) may be calculated. However, the GPS receiver may not be implemented within the mobile station because the GPS receiver consumes a significant amount of power to continuously track the pseudorandom codes from satellites. The battery of the mobile station cannot support such power consumption and also provide power to the mobile station for other standard operations. For cellular terrestrial positioning based on code phases that are derived from correlations with known bit sequences, power consumption is not an issue because the bursts used normally occur in cellular messaging. However, current positioning methods cannot provide an accurate and reliable code phase estimate corresponding to the peak of a sampled ACF.
In a cellular-based application such as Enhanced Observed Time Difference (EOTD) used in Global System for Mobile communication (GSM) based networks and Advanced Forward Link Trilateration (AFLT) used in code division multiple access (CDMA) based networks, range information may be derived from cellular signals and used to determine the location of the mobile station. The code phase of a selected burst sequence corresponding to the peak of an ACF provides the range information.
One aspect of designing a wireless communication system is to optimize resources available to a mobile station. In particular, one method of improving availability of resources within the mobile station is to conserve power and to extend battery life. Therefore, a need exists for an effective means to determine the location of the mobile station without significant increase in power consumption.